It is well known in the art that the noise component in multi-stage optical amplifier output can be reduced by providing optical isolation between gain stages. Such isolation is ordinarily achieved with a spatial filter or alternatively, a saturable absorber. Undesired feedback and (higher) modal impurities are thereby reduced.
The contribution to the noise power which arises from spontaneous emission is ordinarily regarded as an ultimate limit of quantum mechanical origin. Although this is certainly the case, there are aspects to the effects of this noise component which are susceptible of control. The present invention recognizes that the spontaneous emission component present at the output of a gain element has been amplified and that discrimination against the further amplification of an intrinsic spontaneous emission component in each, or following gain stages, reduces the absolute noise at the output.
In the conventional amplifier the optical flux propagates through the gain medium following which a converging lens is used to focus the amplified signal through a selective limiting device such as a spatial filter or saturable absorber. The amplified flux includes a component arising from spontaneous emission in the gain medium, which component is also amplified by the gain medium. The amplified spontaneous emission (hereafter, "a.s.e.") flux is so distributed that a transverse section through the gain medium includes a central region dense in coherent amplified signal (the converging component) in combination with an a.s.e. background and a surrounding "penumbral" region containing only the widely distributed a.s.e. component. In the prior art, a significant portion of the amplified spontaneous emission flux is transmitted through the spatial filter or saturable absorber because both coherent signal and the a.s.e. portion are focused by a lens component provided to assure maximal transmission through the saturable absorber or spatial filter. Consequently the a.s.e. portion is presented to the succeeding gain stage(s) where it is subject to further amplification.
An example of the prior art directed to the study and reduction of a.s.e. in multistage optical amplifiers is the recent work of Migus, et al, "Amplification of Sub-picosecond Optical Pulses: Theory and Experiment", IEEE J. Quant. Elect., QE-18, pp. 101-109 (1982).